In a construction machine provided with a working attachment, wherein the working attachment has a boom, an arm, a working device, a boom cylinder for driving the boom, an arm cylinder for driving the arm, and a working cylinder for driving the working device, a reaction force upon operation acts on the working attachment in such a direction as to extend the boom cylinder, thereby causing a possibility of increasing pressure in a rod-side fluid chamber of the boom cylinder (hereafter, rod-side pressure) and decreasing pressure in a head-side fluid chamber (hereafter, head-side pressure).
This will be explained taking a typical hydraulic shovel shown in FIG. 5 as an example. The hydraulic shovel includes a crawler-type lower traveling body 1 and an upper slewing body 2 installed on the lower traveling body 1 so as to be slewable about an axis perpendicular to the ground. The upper slewing body 2A is attached with an attachment 9. The working attachment 9 has a boom 3, an arm 4, a bucket 5 which is a working device, a boom cylinder 6 that drives the boom 3, an arm cylinder 7 that drives the arm 4, and a bucket cylinder 8 that drives the bucket 5. The working attachment 9 performs various kinds of work such as excavation, loading, leveling and the like as a result of a single operation or compound operation of raising/lowering of the boom 3, pushing/retracting of the arm 4, i.e. upward/downward pivoting, and excavation (scooping) and return of the bucket 5.
FIG. 6 shows a conventional hydraulic circuit for driving the boom cylinder 6 and the arm cylinder 7. The hydraulic circuit includes: a boom remote control valve 10 and an arm remote control valve 11 for operating the boom cylinder 6 and the arm cylinder 7, respectively; a boom control valve 12 and an arm control valve 13 which are respective hydraulic-pilot-operated selector valves adapted to be operated by receiving pilot pressure outputted by the remote control valves 10, 11; and a tank T and a hydraulic pump 14 which are connected to the boom cylinder 6 and the arm cylinder 7 via the control valves 12, 13, respectively.
The boom control valve 12, which is a control valve for the boom cylinder 6, has a neutral position 12n, a boom raising position 12a and a boom lowering position 12b, and is adapted to be switched between these positions to thereby enable the supply and discharge of pressure fluid to/from the boom cylinder 6 to be controlled. Similarly, the arm control valve 13, which is a control valve for the arm cylinder 7, has a neutral position 13n, an arm retracting position 13a and an arm pushing position 13b, and is adapted to be switched between the positions to thereby enable the supply and discharge of pressure fluid to/from the arm cylinder 7 to be controlled. The boom cylinder 6 has a head-side (extension-side) fluid chamber 6a and a rod-side fluid chamber 6b, and the arm cylinder 7 has a head-side (extension-side) fluid chamber 7a and a rod-side fluid chamber 7b. 
During excavation work by the hydraulic shovel through an arm retracting operation alone of the arm 4, an excavation reaction force acting on the working attachment 9 increases the pressure inside the circuits pertaining to the arm cylinder 7 and acts on the boom 3 upward (in the direction of extending the boom cylinder 6) to increase the rod-side pressure of the boom cylinder 6, i.e. the pressure in the rod-side fluid chamber 6b, and decrease the pressure in the head-side pressure, i.e. the pressure in the head-side fluid chamber 6a. This excavation reaction force brings the vehicle body into a raised state to render the continuation of the excavation work impossible.
To avoid such an occurrence, ordinarily performed is a boom raising operation in order to relieve the rod-side pressure of the boom cylinder 6 and an operation of letting out the fluid in the rod-side fluid chamber 6b to the tank T via the boom control valve 12. In short, performed is an arm retracting/boom raising compound operation. This operation reduces the rod-side pressure to thereby enable the boom cylinder 6 to bear the excavation reaction force, allowing the excavation work to be continued.
However, the operation of the boom control valve 12 to the boom raising position 12a causes a meter-in opening and a meter-out opening of the valve 21 to be opened simultaneously, thus allowing a discharge fluid from the hydraulic pump 14 to be flowed into the head-side fluid chamber 6a of the low-pressure boom cylinder 6, that is, allowing the discharge fluid to be supplied to the boom cylinder 6 which essentially requires no fluid. This brings not only energy loss in the circuit but also reduction in the supply amount of the discharge fluid to the arm cylinder 7 (including other hydraulic cylinders such as the bucket cylinder 8 in a case where these other hydraulic cylinders are connected to the hydraulic pump 14) that is driven by the same discharge fluid to reduce power in the arm 4, thereby impairing work efficiency.
The increase in the rod-side pressure and the reduction in the head-side pressure in the boom cylinder 6 occur not only during an arm retracting/boom raising compound operation but also during a compound operation of scooping by the bucket 5 and boom raising for relieving the excavation reaction force derived from scooping. Not limited to the time when the excavation work is performed by the hydraulic shovel, for instance, also when a demolition work is performed by a demolition machine in which an opening and closing-type crushing device called a “nibbler” is attached, in place of the bucket 5, to the leading end of the arm 4, a reaction force during the work can act on the boom cylinder 6 in the direction of extending the cylinder 6, involving the increase in the rod-side pressure and the reduction in the head-side pressure.
As an approach for solving the above problem, Patent Document 1 discloses an art of relieving a rod-side pressure to the head-side fluid chamber of the arm cylinder in a situation of the increase in the rod-side pressure of the boom cylinder and the reduction in the head-side pressure by an excavation reaction force. The art, however, involves narrowing a return passage to a tank from the rod-side fluid chamber of the boom cylinder in order to increase the rod-side pressure of the boom cylinder up to a pressure that can be used as the head-side pressure of the arm cylinder; this gives rise to an adverse effect of generating surplus rod-side pressure in the boom cylinder, upon the extension of the boom cylinder for a work such as horizontal dragging, to increase pressure loss.
Patent Document 1: WO 2004/005727